1. Field of the Invention
This invention relates to the cleaning of a looped forming fabric which is used in a papermaking machine to receive an aqueous slurry of pulp fibers to permit water to be removed therethrough to form the nascent paper web on the outer surface of the looped forming fabric. More particularly, this invention relates to a method and apparatus for cleaning the outer surface and interstices of the looped forming fabric while simultaneously providing for collection of the water mist from spray nozzles used to dislodge pulp fibers from the surface and interstices of the forming fabric.
2. Description of the Prior Art
Prior apparatus and a method for cleaning the forming fabrics in papermaking machines often comprised the application of so-called flooding showers, which applied a relatively high volume-relatively low velocity water to the inner side of the looped forming fabric to loosen and remove contaminants, comprising mostly of paper pulp fibers and so-called fines, from the body, or interstices, of the fabric. Another fabric cleaning apparatus comprised of water showers applying a relatively high velocity-relatively low volume, stream of water to clean contaminants off the outer surface of the looped forming fabric. When these two methods or apparatus were used on the same papermaking machine, the relatively high volume and relatively low volume showers were used at different locations along the length of the forming wire.
Essentially, the relatively high pressure water shower operated by impinging the outer surface of the looped forming fabric to dislodge the wood pulp fibers from the surface of the forming fabric, while the relatively lower pressure water shower operated by impinging the inner surface of the looped forming fabric to flood the forming fabric with a relatively large volume of water which was urged outwardly through the outer surface of the forming fabric to thereby wash wood pulp fibers from the interstices of the forming fabric to thereby clean it.
Regardless of the method and apparatus utilized in prior forming fabric cleaning apparatus, shower mist, in the form of fine water spray, has been a significant problem on papermaking machines since such a mist always contains wood pulp fibers and so-called fines, which are small particles, which might include wood or clay. Mist contaminated with wood pulp fibers or fines is very undesirable because the mist settles on both the machinery itself as well as the paper web being formed. In either case, simply stated, the fibers and fines then deleteriously create a build-up which can become heavy enough to fall off the surface and onto the forming fabric or paper web, resulting in paper defects or sheet breaks during the papermaking process.
Solutions have been proposed to the misting problem. U.S. Pat. No. 5,381,580 teaches that pans, located on the side of the forming fabric opposite to a shower, can be brought into close proximity with the fabric so as to catch and collect water which passes through the fabric under the impetus of the shower. Such pans are assisted by the application of subatmospheric air pressure to more positively remove the water. If several cleaning showers are used, there is need for several individual pans, or at least one very large pan, which in either case requires considerable space.
In addition, the pan-opposite-to-the-shower solution proposed in the U.S. '580 patent only relates to flooding the interstices of the forming fabric to remove the pulp fibers and fines, but does nothing to resolve shower-side misting created when the water from a relatively high-pressure shower impinges the forming fabric.
Another solution utilizes a pair of opposed, relatively high pressure oscillating showers located on either side of the forming fabric. Each such shower is enclosed in a suction box which serves to capture both shower water coming through the fabric, driven by the opposed shower, and the mist created from the shower water impinging the fabric on the same side of the forming fabric. While this solution addresses the misting problem, and provides for a compact unit, the use of only relatively high pressure, so-called needle, showers limits cleaning primarily to the pulp fibers and fines on the surface of the traveling forming fabric.